Finally, a deep drive into the population genetic structure of France, The Genetic History of France:

…These clusters match extremely well the geography and overlap with historical and linguistic divisions of France. By modeling the relationship between genetics and geography using EEMS software, we were able to detect gene flow barriers that are similar in the two cohorts and corresponds to major French rivers or mountains…A marked bottleneck is also consistently seen in the two datasets starting in the fourteenth century when the Black Death raged in Europe.

Nothing too surprising. In a nation of France’s size without strong socio-cultural dynamics that might encourage endogamy, it makes sense that geographic barriers are very important in structure. That being said, there does seem to be a correspondence between deep linguistic differences which date back to antiquity. Additionally, the people of Brittany turn out to be more “British” than not. This is not entirely surprising since the Breton dialect descends from the Brythonic language brought bystanders Celtic Britons (its closest relative is quasi-extinct Cornish).

I do wonder though how much France being a “target” nation for immigration over the centuries has shaped some of these patterns. I’m not talking here about recent non-European immigration, but the migration of Spaniards, Italians, and Poles, in the 19th-century, and earlier. Until the rise of Britain in the 18th-century France had been the largest, most powerful, and in the aggregate wealthiest, Western European nation in the post-Roman world. I suspect that this results in long-term trends toward cosmopolitanism genetically that might be absent in a few populations, such as the French Basque (who are distinct in these data).